This invention pertains to broad spectrum optical fiber radiation sources, and to articles that comprise such sources.
Broadband optical fiber radiation sources are of interest because of their wide range of applications, from sliced spectrum sources to optical noise sources in EDFA (Er-doped fiber amplifier) measurement systems, from optical sensor systems to fiber optic gyroscopes and to low-coherence tomography. In particular, superfluorescent fiber sources (SFS) using amplified spontaneous emission (ASE) from an Er-doped fiber are of considerable interest.
U.S. Pat. No. 5,668,821 discloses use of a long period grating to spectrally flatten the output of a SFS, and U.S. Pat. No. 5,701,318 discloses a polarized SFS, with a polarizer disposed within the superfluorescent fiber. S. P. Parry et al., xe2x80x9cOptical Amplifiers and Their Applicationsxe2x80x9d Conference, TuD3, 1998 disclose a high power/broad band SFS that uses a single long length (51 m) of Er-doped fiber, and J. H. Lee et al., Optics Letters, Vol. 24(5), p. 279, Mar. 1, 1999 discloses a prior art high power/broad band SFS that uses two pumped lengths of Er-doped fiber, and a novel source that uses a single length of pumped Er-doped fiber and an unpumped length of Er-doped fiber before the amplifier stage.
The above recited references are exemplary only. All cited references are incorporated herein by reference.
Various SFS configurations have been reported in the prior art, but the double pass backward (DPB) configuration has been shown to provide the largest bandwidth and the highest conversion efficiency. However, even using a DPB configuration, it is difficult to devise a SFS that has high power (e.g., greater than 10 mW) and broad bandwidth (e.g., greater than 40 nm between xc2x13 dB points).
The main difficulty with achieving high power and broad bandwidth is the bandwidth narrowing that is experienced at high pump powers by SFSs. See, for example, S. P. Parry et al., op.cit. It is known that this bandwidth narrowing is a consequence of the wavelength-dependent gain coefficient of Er-doped fiber. In view of the great utility of high power/large bandwidth SFSs, it would be desirable to have available a broadband SFS (e.g.,  greater than 40 nm between xc2x13 dB points) that can provide high power (e.g.,  greater than 10 mW). This is because increased SFS bandwidth typically results in increased resolution in, e.g., an optical tomography system, and increased power (spectral density) typically results in increased signal to noise ratio. This application discloses such a broad band/high power SFS.
By xe2x80x9clightxe2x80x9d we mean herein electromagnetic radiation of wavelengths of interest for SFSs, generally in the infrared part of the spectrum.
The xe2x80x9crare earthsxe2x80x9d (REs) are the elements of atomic numbers 57-71, and the rare earths that are suitable for stimulated emission in a silica-based fiber will be referred to as xe2x80x9cSE-REsxe2x80x9d. Preferred SE-REs are Er, Yb and Nd.
In a broad aspect, the invention is embodied in a SFS of novel design, and in articles (e.g., a communication system, a measurement system, an optical sensor system, a fiber optic gyroscope, a low-coherence tomography system) that comprise the SFS.
More specifically, the invention is embodied in an article that comprises an optical fiber light source. The light source comprises a first and a second length of SE-RE-doped optical fiber, disposed such that light can be transmitted axially from the first to the second length. The light source also comprises a source of first pump light, and also comprises a coupler for coupling first pump light into the first length of SE-RE-doped optical fiber into the downstream direction.
Significantly, the light source also comprises a source of second pump light, a coupler for coupling the second pump light into the second length of SE-RE-doped optical fiber into the upstream direction, and an optical isolator disposed between the first and second lengths of SE-RE-doped optical fiber such that upstream-propagating light from the second length of SE-RE-doped optical fiber is substantially prevented from reaching the first length of SE-RE-doped optical fiber. In a preferred embodiment, the light source furthermore comprises a reflector disposed to reflect at least some upstream-propagating light back into the first length of SE-RE-doped optical fiber in the downstream direction, whereby generation of long-wavelength ASE is facilitated. The long-wavelength ASE is transmitted through the optical isolator from the first to the second length of SE-RE-doped optical fiber, where broadband ASE is generated. The broadband ASE propagates from the second length of SE-RE-doped optical fiber in the downstream direction to utilization means.